PAPI overhead on various kernel interfaces

Note these results are *extremely* old. For more recent results please see here.

Older results

These are the results of running the "papi_cost" test program with PAPI on a Core2 machine running four different kernels. The reported value is "average cycles", a smaller value is better.
KernelStart/StopReadRead TSAccumReset
2.6.32-pe 17749 8982 9019 11519 2550
2.6.35-pe 8683 2089 2109 4196 2084
2.6.30-perfmon 9644 1200 1202 3623 2406
2.6.32-perfctr 5702 195 203 3772 3556

The poor behaviour on 2.6.32/perf_events is due to various workarounds that need to be done to properly set up the counters and read out the data. These were fixed by 2.6.35.
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